A catalytic converter for eliminating unwanted components in emissions is conventionally provided in an exhaust passage of an engine. Examples of the catalytic converters include oxidation catalysts composed of catalytic substances, such as catalytic metals, e.g., precious metals or transition metals on supports for oxidizing substances such as hydrocarbons or carbon monoxides in emissions and filters for collecting and eliminating particulate matter in emissions. In such oxidation catalysts of the catalytic converters, the supported catalytic substances exhibit oxidation characteristics at temperatures that activate the catalytic substances. At a low emission temperature of the engine, that is, at a low temperature of the catalyst, the oxidation catalyst cannot sufficiently purify the emissions. Specifically, diesel engines have low emission temperatures as compared to gas engines, and thus the temperature of the oxidation catalyst is left at low temperatures for long hours.
As disclosed in Japanese Unexamined Patent Application Publication 11-82003, the oxidation catalysts generally include substances that can adsorb hydrocarbons (HCs) in emissions at low temperatures, such as zeolite or alumina. Oxidation catalysts including such substances are referred to as HC trapping catalysts or adsorption oxidation catalysts and can adsorb HCs in the emissions even at a low temperature of the catalyst to clean the emissions. At a high temperature of the catalyst, adsorbed HCs are desorbed from the catalyst to be oxidized by the catalytic substance and eliminated.
Unfortunately, such a catalytic converter including the oxidation catalysts gradually deteriorates during repeated use, which lowers the performance to purify emissions. To keep an appropriate level of cleaning of emissions, accurate determination of the degree of deterioration of the catalytic converter is necessary. Examples of the approach that determines the deterioration of oxidation catalysts capable of oxidation and both adsorption and desorption of HCs are disclosed in Japanese Unexamined Patent Application Publication 2006-118358 and Japanese Patent No. 4466451.
Determination of the deterioration of the oxidation catalysts that can perform both oxidation and adsorption/desorption of HCs requires appropriately specifying the factor causing the deterioration, i.e., a decrease in oxidation performance due to deterioration of the catalytic substance or a decrease in the adsorption/desorption performance due to deterioration of the adsorbable substance.
Unfortunately, the adsorption substance adsorbs HCs at a low temperature of the catalyst; hence, a technique to monitor changes in the temperature of the oxidation catalyst as disclosed in Japanese Unexamined Patent Application Publication 2006-118358 is not suited to such determination of the deterioration of oxidation catalysts. A technique that performs specific control for the determination of deterioration of the catalyst is applicable as is disclosed in Japanese Patent No. 4466451; however, it brings out a demand for a simpler way to determine the deterioration.